1. Field of the Invention
The present invention is a keyboard interface circuit with a bi-directional bus connectable to a separate central processing unit (CPU) and, more particularly, the present invention is directed to a keyboard which multiplexes a key code to a CPU and light codes from the CPU over the same bi-directional bus to provide feedback to a keyboard operator concerning key selection.
2. Description of the Related Art
In many applications, particularly in the field of nuclear power plant control, it is necessary to backlight keys which have been selected and which correspond to particular dedicated displays. The keys are backlit to provide feedback to the operator that indicates that the proper display is being displayed.
Prior art keyboards as described above are of one of two types where each type has a set of read lines for the separate CPU to read key selections and a set of write lines for the separate CPU to light the corresponding light.
The first type has the same number of read lines as keys and the same number of write lines as lights. That is, if the keyboard has 64 keys and 64 lights, the interface bus includes 128 lines. Many computers, particularly computers connected to other devices besides the keyboard, do not have sufficient input/output lines to dedicate a line for each key and a corresponding line for each light.
In an attempt to reduce the number of lines, many keyboard manufacturers are installing dedicated keyboard computers in the keyboard connected to the keys and lights and to the separate CPU. The keyboard computer transfers key codes over a read bus to the separate CPU and the separate CPU transfers the corresponding light codes back to the keyboard CPU over a write bus. If the keyboard includes 64 keys and 64 lights a read bus with at least 6 lines and a write bus with at least 6 lines is necessary. This configuration reduces the number of read and write lines from 128 to 12, however, the complexity and cost of the keyboard rises due to the addition of the keyboard computer and the necessary software for controlling same.